Hi , I`m JackPerhaps I can help you with this problem , a few questions coming up
Once turned on and warmed up ( 1 hour or so) , can you turn it off and right back on normally ?
Sorry but i was out last night when you replied ,it was 11:15 PM here .
You did not give me the model number of the tv but the symptoms you described and the fact that panasonic hasn`t made a plasma tv for 5 + years tells me its not a young tv and what you have is a typical case of weak/vented capacitors on the power board .
Around here you would be looking at some $250 to have this repair in a shop .
If you are into this you could simply replaced the power board VS repairing it , that board usually cost somewhere around $100 but one detail on this ; the VS and VA voltages on the power board will have to be adjusted according to the values written at the back of your plasma panel , they are specific to each panel and it will be on a sticker .
You will need a voltmeter for this .
Again if you are into this then let me have the TV's model number and I`ll see what I can find otherwise if you are going to go for the repair shop or even a new tv then never mind the model number
Let me know if you have other questions and if not for now then please do not forget to rate by clicking on the star icons at the top, then click on Submit so i get paid for my time helping you .Also note that it will not close this post , you can come back here at any time if you need further info or help . Thank you